Diffusion of Al atoms and growth of Al nanoparticle clusters on surface of Ni substrate

Abstract

NiAl nanoparticles possess high-energy density and good mechanical properties at elevated temperatures, and are considered as an important material. However, the differences in the diffusion behavior of Al adsorbed atoms on different Ni substrate surfaces and the effects of different diffusion mechanisms on the deposition growth of Al atoms on the Ni substrate surface are highly desired to be clarified. Therefore, in the present work, the diffusion behavior of single Al adsorbed atoms and nanoparticle cluster growth on the Ni substrate surface of decahedral (DEC), cuboctahedral(CUB) and icosahedral(ICO) structures are systematically studied by molecular dynamics (MD) throuh analyzing the embedded atom potentialand using the nudged elastic band method. The diffusion barriers of Al adsorbed atoms on three different Ni substrates are calculated by nudged elastic band methodand analyzed, showing that the diffusion barrier is greatly affected by the smoothness of the step edge and the atomic coordination number of substrate as well. The diffusions of Al adsorption atoms on the surfaces of three Ni substrates are realized by two mechanisms, namely exchanging or hoping, and the lowest Ehrlich-Schwoebel (ES) barrier is 0.38 eV for exchange CUB{111} → {100}, 0.52 eV for exchange DEC{111} → {100}, and 0.52 eV for hoping ICO {111} → {111}. The exchanging mechanismsupports Al adatoms diffusing from {111} to {100} facet on the three Ni substrates, while the diffusion between two adjacent {111} facets is mainly driven by the hoping mechanism. On this basis, atom-by-atom growth MD simulation is used to study the structure of the Ni-Al cluster. The deposited Al atoms first tend to diffuse near the edges of the steps and the vertices. The deposited Al atoms begin to aggregate into islands with the increase of their number. For Al atoms on the Ni cluster, a good Ni-core/Al-shell structure can be obtained by depositing Al atoms on the surface of Ni substrate at lower temperatures. In this core-shell structure, Al atoms have a larger surface energy and atom radius compared with Ni atoms. For the ICO substrate, the corresponding defect number of core-shell clusters is smaller than for the CUB and the DEC substrate, which is in good agreement with the diffusion behavior of Al adsorbed atoms on the Ni substrate cluster surface. The surface of Ni-Al bimetal is gradually alloyed with the increase of growth temperature. This study provides a good insight into the diffusion and growth of Al adsorbed atoms on Ni substrates surface on an atomic scale.